In avalanche photodiodes or photodetectors, incoming light is used to generate carriers (i.e., free electrons or holes). Once the avalanche breakdown begins, the carriers are accelerated by the electric field to very high speeds striking other atoms and knocking carriers free from another atom, and ionizing it. As this process continues the number of free carriers moving through the material increases exponentially; often in just picoseconds. The avalanche multiplication process takes place in the multiplication portion of the photodetector. The carriers are absorbed for conversion to electrical current in the absorption portion of the photodetector.
An avalanche photodiode (APD) semiconductor device has a built-in first stage of gain through avalanche multiplication. Using a high reverse bias voltage (typically 100-200 V in silicon), APDs show an internal current gain effect (around 100) due to impact ionization, which is commonly referred to as avalanche effect. Gain in an avalanche device is achieved through photogenerated charge carrier impact ionization and a resultant multiplication of total charge carriers available. Typical applications for APDs include laser rangefinders and long range fiber optic telecommunication. Parameters for judging the usefulness of APDs for a particular application include quantum efficiency, or the efficiency related to the absorption of incident optical photons and subsequent generation of primary charge carriers; total dark current, noise equivalent power, spectral sensitivity range and operating voltage.
Semiconductor materials are selected for photodiodes based upon the wavelength range of the radiation that is desired to be utilized or detected. Group III-nitride avalanche detectors can presumably be widely functional between 1900 nm to 200 nm (i.e. infrared radiation to ultraviolet). Generally, the binaries utilized in such semiconductor devices are Indium Nitride (bandgap of 0.65 eV corresponding to approximately 1900 nm), Gallium Nitride (band gap of 3.4 eV corresponding to approximately 365 nm) and Aluminum Nitride (bandgap of 6.1 eV corresponding to approximately 200 nm). By varying the relative mole fractions of the binaries, ternary or quaternary alloys may be composed that can achieve radiation absorption at intermediate wavelengths to the stated values.
U.S. Pat. No. 6,326,654 to Ruden in (hereinafter Ruden '654; hereby incorporated by reference) entitled “A Hybrid Ultraviolet Detector,” discloses what appears to be a semiconductor material avalanche photodiode photodetector. However, Ruden '654 does not take advantage of the polarization properties of III-Nitride semiconductor material; in fact it is completely ignored.